Within self-consistent field theory we study the phase behavior of a
symmetrical binary AB polymer blend confined into a thin film. The film
surfaces interact with the monomers via short range potentials. One surface
attracts the A component and the corresponding semi-infinite system exhibits a
first order wetting transition. The surface interaction of the opposite surface
is varied as to study the crossover from capillary condensation for symmetric
surface fields to the interface localization/delocalization transition for
antisymmetric surface fields. In the former case the phase diagram has a single
critical point close to the bulk critical point. In the latter case the phase
diagram exhibits two critical points which correspond to the prewetting
critical points of the semi-infinite system. Only below a triple point there is
a single two phase coexistence region. The crossover between these
qualitatively different limiting behaviors occurs gradually, however, the
critical temperature and the critical composition exhibit a non-monotonic
dependence on the surface field. The dependence of the phase behavior for
antisymmetric boundaries is studied as a function of the film thickness and the
strength of the surface interactions. Upon reducing the film thickness or
decreasing the strength of the surface interactions we can change the order of
the interface localization/delocalization transition from first to second. The
role of fluctuations is explored via Monte Carlo simulations of a coarse
grained lattice model. Close to the (prewetting) critical points we observe 2D
Ising critical behavior. At lower temperatures capillary waves of the AB
interface lead to a pronounced dependence of the effective interface potential
on the lateral system size.Comment: submitted to the Journal of Molecular Liquids and Condensed Matter
Physic
